NiTi alloys with nearly equiatomic chemical composition are the most studied and the most practically utilized materials from the group of the shape memory alloys (SMA). The NiTi alloy has a thermoelastic martensitic transfor- mation (MT), which its two most important properties are based on: the shape memory and the superelasticity. Thin films of NiTi alloys with thicknesses from hundreds of nm to units of µm have a wide use mainly in microelectromechanical systems (MEMS). The doctoral thesis deals with a fabrication of thin NiTi films using a deposition on silicon substrates with two techniques - the magnetron sputtering and the pulsed-laser deposition (PLD), an investigation of their mi- crostructure using x-ray diffraction, a heat treatment of amorphous NiTi films and with a verification of MT using resistometry. Furthermore, thermomechan- ical properties of a system NiTi-polyimide (NiTi-PI), namely a dependence of a curvature radius of a NiTi-PI bilayer on temperature, are studied. 1

Many industries including the medical, aerospace, and automobile industries have increasingly adopted the use of shape memory alloys (SMAs) for a plethora of applications due to their unique thermomechanical properties. From the commercially available SMAs in the market, binary NiTi SMAs have shown the most desirable properties. However, SMA properties can be significantly affected by the fabrication process. One of the most familiar applications of NiTi SMAs is in the design of actuating devices where the shape memory effect properties are highly advantageous. Spring NiTi SMA actuators are among the most commonly used and are generally made by torsion loading a straight wire. Consequently, stress concentrations are formed causing a reduction in recovery force. Other methods for producing springs and other NiTi SMA components is the fast emerging manufacturing method of additive manufacturing (AM). AM often uses metal powders to produce the near-net shape components. A major challenge for SMAs, in particular, is their well-known composition sensitivity. Therefore, it is critical to control composition in NiTi SMAs. In this thesis, a novel method for processing NiTi SMAs for pre-alloyed NiTi SMA powders and springs is presented. A low pressure and low temperature hydriding-pulverization-dehydriding method is used for preparing the pre-alloyed NiTi SMA powders with well-controlled compositions, size, and size distributions from wires. By hydrogen charging as-drawn martensitic NiTi SMA wires in a heated H3PO4 solution, pulverizing, and dehydriding, pre-alloyed NiTi powders of various well-controlled sizes are produced. In addition, a low pressure and low temperature hydriding-dehydriding method is used for producing NiTi SMA helixes from wires. The helix pattern in the pre-alloyed NiTi SMA wires was obtained by hydrogen charging NiTi SMA 500 μm diameter wires at different time intervals, followed by dehydriding to remove the hydrogen. The wires, powders, and resulting helixes were characterized using scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and x-ray diffraction (XRD). The relationship between the wire diameter, powder particle size, and helix geometry as a function of hydrogen charging time is investigated. Lastly, the recovery behavior due to the shape memory effect is also investigated after dehydriding.

No description available.

One of the main issues with the use of nickel titanium alloy (NiTi) implants in cardiovascular implants (stents) is that these devices must be of very high quality in order to avoid subsequent operations due to failing stents. For small stents with diameters below ca. 2 mm, however, stent characterization is not straightforward. One of the main problems is that there are virtually no methods to characterize the interior of the NiTi tubes used for fabrication of these tiny stents. The current paper reports on a robust hybrid actuator for the characterization of NiTi tubes prior to stent fabrication. The method is based on a polymer/hydrogel/magnetic nanoparticle hybrid material and allows for the determination of the inner diameter at virtually all places in the raw NiTi tubes. Knowledge of the inner structure of the raw NiTi tubes is crucial to avoid regions that are not hollow or regions that are likely to fail due to defects inside the raw tube. The actuator enables close contact of a magnetic polymer film with the inner NiTi tube surface. The magnetic signal can be detected from outside and be used for a direct mapping of the tube interior. As a result, it is possible to detect critical regions prior to expensive and slow stent fabrication processes.

Ni50.6Ti49.4 single and Ni52Ti48 polycrystalline shape memory alloy samples were subjected to aging under a uniaxial stress, to form a single Ni4Ti3 precipitate variant and to investigate the effects of single versus multi-variant coherent precipitates on the shape memory characteristics including two-way shape memory effect (TWSME). Shape memory and superelasticity properties along with the effects of stress and temperature on the transformation temperatures, strain, hysteresis, dimensional stability, and R-phase formation were investigated. This was accomplished through the use of isobaric thermal cycling and superelasticity experiments and various microscopy techniques that included transmission electron microscopy (TEM), scanning electron microscopy, and optical microcopy. The results showed that it is feasible to use constrained aging to bias R-phase martensite variants upon cooling from austenite without any external stress, however, accomplishing this with B19’martensite was much harder as complete TWSME was only found in the Ni50.6Ti49.4 single crystalline sample oriented along the [112] direction. The onset of irrecoverable strain corresponded to the R-phase temperature hysteresis increase in the single crystalline samples regardless of the aging conditions. Through TEM analysis it was discovered that [112] and [114] twins were found in austenite due to plastic deformation of martensite during the superelasticity experiments. Since [112] twins are theoretically impossible to form in austenite, and since martensite was plastically deformed, [112] austenite twins were attributed to the transformation of compound twins in martensite, in particular [113] martensite twins formed during the plastic deformation of martensite, into austenite twins. In the Ni52Ti48 polycrystalline samples, a compressive R-phase variant was biased through constrained aging under 100 and 200 MPa uniaxial tensile stresses at 400°C and 450°C. Aging, in all conditions, produced a high density of Ni4Ti3 precipitates that was most likely responsible for the small transformation strain observed, less that 2%, upon transformation to martensite. In the future, samples with compositions between 50.8 and 51.5 Ni atomic percent, in addition to altered solution and aging heat treatments as compared to those used in this study should be investigated as it is believed that samples with these compositions will yield better and consistent TWSME responses through constrained aging.

Cardiovascular diseases which include high blood pressure, coronary heart diseases, heart failure, stroke and peripheral arterial diseases (PAD) affect one out of three American adults or 105 million people. By 2030, the prevalence of cardiovascular disease is estimated to rise 10 percent to more than 40 percent of American adults, or 116 million people. Approximately 8 million people in the United States have PAD, including 12-20% of individuals older than age 60. The main reason of PAD is obstruction of blood flow through lower extremities causing Atherosclerosis. The major artery affected in the lower extremities due to PAD is superficial femoral artery (SFA). Huge numbers of clinical procedures like superficial femoral artery stenting, balloon angioplasty, and percutaneous transluminal intervention are done to treat the disease. Thorough in vitro (biological phenomena made to occur outside the human body) testing of this kind should reduce the risk of in vivo (biological phenomena occurring inside the human body) stent failure and thus lead to increased survivability for patients suffering with cardiovascular diseases and PAD. It has been recognized that a metal subjected to a repetitive or fluctuating stress will fail at a stress much lower than that required to cause failure on a single application of load. As per literature review, typically SFA stents survive no more than 12-18 months until the first fracture is detected in MRI. There was a need of a customized designed device such that it would simulate realistic blood pressure conditions and test the capacity the SFA stents. Commercially, the stents are tested under accelerated cyclic loading conditions at different frequencies for longer cycle periods. In order to demonstrate how stents perform once deployed into an artery, a testing device was required which will simulate arterial blood pressure variations and compressive loads over artery as close as possible to human body. The stent tester documented in this report is capable of subjecting a stent to appropriate physiological loading by deploying it in a simulated vessel and subjecting it to external compression. Loading of this kind was performed at frequencies at 60 Hz and, as such, simulating one million heartbeats of artery pulsation. The primary purpose of this thesis was to successfully design stent tester which cycles artificial fluid simulating blood pressure in arteries and superimposing cyclic compression of stent deployed in an artificial artery. The goal was to obtain a test machine that allows for a cost effective testing of cardiovascular and peripheral stents. Another goal was to externally compress the arterial wall subjected to compressive load with the help of an air controlled mechanical piston attached with load cell assembly. The load cell measures the amount of load applied over the silicone tubing. The design of the device contains two peristaltic pumps which alternate pressure every second by pumping distilled water via plastic tubing. The stent was crimped with the guide wire catheter and deployed in the silicone artery (diameter 11mm O.D) from Dynatek labs. Wall and bridge stents (Medtronic, Schneider Inc.) 10 x 39 mm were used for testing with and without external load. Industrial pressure transducer S-10 (Wika Instruments Corporation) ranging from 0-5 PSI (0-259mmHg) is used to monitor the pressure in the artificial artery. The pressure transducer is connected to the data logger (Omega om320) which serially communicates with computer. The silicon mock test artery was 20cm long so that stents used in various arterial interventions can be tested. The silicon artery has primary advantages over rubber latex artery which are clarity and close resemblance to human artery and durable (ideal for long term durability tests). Preliminary results from literature review show that stent materials, based on its mechanical properties survive for more than one million heartbeats. To demonstrate the capacity of current design a nitinol stent was tested under physiological conditions at 60Hz frequency. A load of 980 grams for 2001 - 5000 heartbeats, 1.98 kg for 5001 - 10000 heartbeats, 4.25 kg for 10001 - 25000 heartbeats and 6.54 kg for 25001 - 50000 heartbeats was applied over the artificial artery. The mechanical piston with load cell assembly was allowed to externally compress the artificial artery. Partial functionality of device was demonstrated by running it for one million heartbeats and 48000 compressive cycles. The device was successfully designed and has the capacity to cycle artificial fluid simulating blood pressure changes in arteries and have demonstrated the ability to test any type of stents. The device was designed efficient which was simulated in an acceptable pressure range as compared to human blood pressure and allow for compression of stent in a cyclic testing pattern. The system was maintained to as close as between diastolic value of 76mmHg to diastolic 122mmHg pressure range. The device was run for about one million heartbeats and it was observed that the NiTi stent successfully survived. The stent was observed visually with a magnifying glass for any cracks or failure at intervals of 500, 1000, 2000, 5000, 10000, 25000, 50000 and one million heartbeats respectively. After running the device for one week at a frequency of 60Hz, no fractures on the stent were visually observed. However, the stent was deformed from the center. Data analysis showed that the mean diastolic and systolic pressure measurements for intervals with no load were found to be statistically significant i.e. in acceptable range. However, the device design lacks stability due to various reasons like device operates in an open looped system, has bubbles in the artificial artery which might be producing varying pressure values and variation in applying load. The device was partly unable to simulate arterial blood pressure changes under no loading conditions. Efforts are being made to improve the design of the device to make it realistic simulation of variation in arterial blood pressure for long term durable testing of the stents.

Devido a suas propriedades, de resistência a corrosão, boa resistência à fadiga, memória de forma, superelasticidade e biocompatibilidade, a liga de NiTi através de seus subprodutos, é amplamente utilizada em diversas áreas médicas, com destaque na confecção de órteses endovasculares. Este trabalho tem como objetivo comparar propriedades de amostras produzidas via metalurgia do pó convencional na obtenção da liga NiTi, utilizando duas diferentes misturas preparadas em moinho atritor, uma fazendo uso de pó de titânio comercial e outra utilizando hidreto de titânio, com vista a aumentar a formação da fase NiTi. Essas amostras foram sinterizadas em forno resistivo a vácuo e em forno assistido a plasma. As amostras sinterizadas apresentaram majoritariamente as fases NiTi e NiTi2. As amostras que fizeram uso de hidreto de titânio tiveram uma quantidade de poros excessiva, levando a uma queda da dureza se comparada a amostra que fez uso de titânio metálico. A mistura de pós sinterizada no forno assistido a plasma sofreu síntese por combustão apresentando poros grandes na escala milimétrica, mas alta integridade na região metálica devido a formação de fase líquida. / Due to its properties like corrosion resistance, good fatigue strength, shape memory, superelasticity and biocompatibility, Nitinol is commonly used in different areas, especially in the manufacturing of stents. This work had as aim to compare properties of NiTi samples produced by conventional powder metallurgy in obtaining NiTi alloys. The different mixtures prepared in atritor mill, making use of commercial titanium powder and other using titanium hydride, to increase the NiTi phase formation were employed. These samples were sintered in a resistive furnace and plasma sintering reactor. The sintered samples showed the phases NiTi and NiTi2. Samples with titanium hydride had a greater quantity of pores, leading to a decreased hardness when compare to samples with titanium metal powder. The samples in the plasma sintering reactor were sintered by combustion synthesis.

Devido a suas propriedades, de resistência a corrosão, boa resistência à fadiga, memória de forma, superelasticidade e biocompatibilidade, a liga de NiTi através de seus subprodutos, é amplamente utilizada em diversas áreas médicas, com destaque na confecção de órteses endovasculares. Este trabalho tem como objetivo comparar propriedades de amostras produzidas via metalurgia do pó convencional na obtenção da liga NiTi, utilizando duas diferentes misturas preparadas em moinho atritor, uma fazendo uso de pó de titânio comercial e outra utilizando hidreto de titânio, com vista a aumentar a formação da fase NiTi. Essas amostras foram sinterizadas em forno resistivo a vácuo e em forno assistido a plasma. As amostras sinterizadas apresentaram majoritariamente as fases NiTi e NiTi2. As amostras que fizeram uso de hidreto de titânio tiveram uma quantidade de poros excessiva, levando a uma queda da dureza se comparada a amostra que fez uso de titânio metálico. A mistura de pós sinterizada no forno assistido a plasma sofreu síntese por combustão apresentando poros grandes na escala milimétrica, mas alta integridade na região metálica devido a formação de fase líquida. / Due to its properties like corrosion resistance, good fatigue strength, shape memory, superelasticity and biocompatibility, Nitinol is commonly used in different areas, especially in the manufacturing of stents. This work had as aim to compare properties of NiTi samples produced by conventional powder metallurgy in obtaining NiTi alloys. The different mixtures prepared in atritor mill, making use of commercial titanium powder and other using titanium hydride, to increase the NiTi phase formation were employed. These samples were sintered in a resistive furnace and plasma sintering reactor. The sintered samples showed the phases NiTi and NiTi2. Samples with titanium hydride had a greater quantity of pores, leading to a decreased hardness when compare to samples with titanium metal powder. The samples in the plasma sintering reactor were sintered by combustion synthesis.

Devido a suas propriedades, de resistência a corrosão, boa resistência à fadiga, memória de forma, superelasticidade e biocompatibilidade, a liga de NiTi através de seus subprodutos, é amplamente utilizada em diversas áreas médicas, com destaque na confecção de órteses endovasculares. Este trabalho tem como objetivo comparar propriedades de amostras produzidas via metalurgia do pó convencional na obtenção da liga NiTi, utilizando duas diferentes misturas preparadas em moinho atritor, uma fazendo uso de pó de titânio comercial e outra utilizando hidreto de titânio, com vista a aumentar a formação da fase NiTi. Essas amostras foram sinterizadas em forno resistivo a vácuo e em forno assistido a plasma. As amostras sinterizadas apresentaram majoritariamente as fases NiTi e NiTi2. As amostras que fizeram uso de hidreto de titânio tiveram uma quantidade de poros excessiva, levando a uma queda da dureza se comparada a amostra que fez uso de titânio metálico. A mistura de pós sinterizada no forno assistido a plasma sofreu síntese por combustão apresentando poros grandes na escala milimétrica, mas alta integridade na região metálica devido a formação de fase líquida. / Due to its properties like corrosion resistance, good fatigue strength, shape memory, superelasticity and biocompatibility, Nitinol is commonly used in different areas, especially in the manufacturing of stents. This work had as aim to compare properties of NiTi samples produced by conventional powder metallurgy in obtaining NiTi alloys. The different mixtures prepared in atritor mill, making use of commercial titanium powder and other using titanium hydride, to increase the NiTi phase formation were employed. These samples were sintered in a resistive furnace and plasma sintering reactor. The sintered samples showed the phases NiTi and NiTi2. Samples with titanium hydride had a greater quantity of pores, leading to a decreased hardness when compare to samples with titanium metal powder. The samples in the plasma sintering reactor were sintered by combustion synthesis.

No description available.